A dictum of golf technique is that the club face should remain parallel to and on the swing plane except near the impact point if the desired delayed hit is to be achieved. This preceding dictum means that the golfer must rapidly rotate the club shaft 90 degrees about its longitudinal axis in approximately 58 milliseconds just prior to impact given a club head velocity of 75 MPH. The delayed hit which is essential to a good golf swing requires the golfer to : (a) maintain a cocked wrist position until approximately the last 90 degrees of swing arc just prior to impact, (b) while in this cocked wrist position, the club face must be parallel to and on the swing plane, (c) and then in approximately 58 milliseconds, just prior to impact, rotate the club shaft so that the club face is perpendicular to the swing plane. The center of gravity of the club head which is approximately at the mid-point of the club head between the heel and toe extremities must be moved off of the swing plane by rotating the club shaft 90 degrees about its longitudinal axis in 58 milliseconds. The rotation of the club shaft is accomplished by the use of the golfer's pronator and supinator wrist muscles. The club head center of gravity, is experiencing high centrifugal forces during this critical 90 degrees (last arc segment before impact) of the swing arc.
Centrifugal force can be defined as, a.sub.n =v.sup.2 /r. If we let r=4.04 feet and v=110 feet/second (75 MPH), we are looking at a centrifugal force of approximately 93 Gs. Due to the movement of the pivot point during the golf swing, it is expected that the G force would be considerably less than 93, but even with a G force of 50, the golfer is compelled to rotate the club shaft 90 degrees about its longitudinal axis in 58 milliseconds against this high G force which is resisting any reaction to move the club head center of gravity off of the swing plane. It is the wrist muscles, mainly the left wrist supinator muscle and the right wrist pronator muscle, that provides the reaction to rotate the club shaft about its longitudinal axis. An authority states that: "Every good golfer has his left wrist in this supinating position at impact." [Ben Hogan in Five Lessons The Modern Fundamentals Of Golf, pp 101, 102; A Fireside Book, Simon & Schuster, Inc. New York Copyright 1957 by Ben Hogan]. It is important to note that the authority Ben Hogan pages 101 and 103 strongly advises against pronation of a golfer's left wrist just before impact.
It is the intent of this invention's golf swing trainer to enhance the development of the golfer's left wrist supinator muscle and right wrist pronator muscle and the other muscles required to perform a good golf swing. The pronator muscle is defined as the right wrist muscle required to rotate the club shaft counterclockwise (for a right handed golfer) about its longitudinal axis, just prior to impact. The supinator muscle is defined as the left wrist muscle required to rotate the club shaft counterclockwise about its longitudinal axis, just prior to impact. Hereinafter, the pronator and supinator muscles maybe referred to as the delayed hit muscles. The delayed hit has already been defined as a special case condition existing through the last 90 degrees of the swing arc just prior to impact. Hence, there is a need for the golfer to develop his delayed hit muscles if he is to consistently achieve the delayed hit which is so essential to a good golf swing.